It is often desirable to accurately monitor subsidence and/or rising of a waterbottom for example in an area with tectonic activity and/or where crude oil and/or natural gas is produced from an underwater oil and/or natural gas containing formation.
In such case it is desirable to measure subsidence and/or rising of a large area of the waterbottom with high resolution and accuracy in the order of centimeters, or better. It would be possible to install an array of pressure sensors, which measure the ambient water pressure at the waterbottom to monitor subsidence and/or rising thereof.
However, tidal variations of waterdepth, variations of atmospheric pressure, waves and currents will generate significant variations of the measured pressure and if the pressure sensors are installed in a deepwater area, for example at a waterdepth of several kilometers the pressure sensors must be capable of withstanding pressure variations of several hundreds of Bar, whereas they must measure pressure variations with an accuracy in the order of 0.001 Bar in order to monitor a vertical waterbottom movement in the order of one centimeter.
It is known from French patent 2661744 to measure seabed subsidence with a series of pressure sensors arranged in a pressure resistant tube which is filled with a low pressure fluid. Other subsidence monitoring systems are known from Japanese patent 60235012, French patent 2749473, U.S. Pat. No. 4,145,817, UK patent 2224845, International patent application WO01/42818.
A problem with the system known from French patent 2661744 is that during installation the hydrostatic pressure of the fluid within the tube may damage the highly sensitive pressure sensors.
It is an object of the present invention to provide a method and system for monitoring subsidence and/or rising of a waterbottom using adequately protected pressure sensors that are able to monitor any vertical waterbottom movements with an accuracy in the order of centimeters even at large waterdepth of up to several kilometers and/or in an area with significant currents, tidal depth variations and/or wave impact, and wherein the tube can be installed at a significant waterdepth without the risk of damage to the pressure sensors due to the hydrostatic pressure of the fluid within the tube.